Bitz are your friend here. I recently set about making some small, basic objective markers for my IG army, using extra bitz off a bunch of random sprues I had lying around. I made one with a small pile of greenstuff with a bunch of grenades and a spare grenade launcher ammo box (the greenstuff below it to make it look like a larger pile, with the TNT on top where you can see it), to represent an ammo dump, and stuck it all on a spare small round base. I made another by taking 2 vox units and putting them back to back (with a little clipping/cleaning to make them fit better) and put a little radio dish thing I had off one of my vehicle spure (I think it was probably from a rhino or something, no clue) to make it look like some kind of beefed up communications unit.
I made a handful of others, but you get the gist of it. I imagine for ork ones you could probably make some kind of bosspole looking thing. I haven't gotten an ork kit since they came with 16 "space ork boys" so I'm not sure what kind of bits come in them these days, but you could get some popsickle sticks or something, break them up to look more orky, and put the top of the bosspole on the top of it (so it's tall enough to scrap over; no ork would want a small trophy, after all). You could also try getting as many spare guns/tank parts/bit of metal/etc, to make it look like something a mek would love to get his grubby hands on, for a more general-purpose marker.
